NodeJS - module.export & fileRead 值返回给调用者 Class

Question

I have readmsgfile.js file which has module.exports and fs.readFile function in it which reads a File and returns the data after some operation.

I would like to return the msgBody from readmsgfile class but the Caller Class A always gets "Undefined" in the line code var respondMsg . I tried to return the value outside of fs.readfile scope but did not work.

what modifications have to be done to get the msgBody var value from readmsgfile file to caller Class A

readmsgfile.js
--------------------
const fs = require("fs");

var msgBody;

module.exports = {
  readMSGFile: async function (filepath) {

   let path = filepath;
    var regex = /\\/g;
    const pathOfString = path.replace(regex, "/");

      fs.readFile(pathOfString, function (err, data) {
      var msgBody = Sum Operation Done! ;
      return msgBody;
    });

   return msgBody;
  },
  
};


Class A:
--------------------------
const readFile = require("../readmsgfile");

Async function(){
var respondMsg = await readFile.readMSGFile(detailsoffile);
console.log(respondMsg);
}

Answer 1

You should not mix callbacks and async functions up:

Promise way:

//readmsgfile.js
module.exports = {
  readMSGFile: function (filepath) {
    // returning the Promise
    return new Promise((resolve, reject) => {
      let path = filepath;
      var regex = /\\/g;
      const pathOfString = path.replace(regex, "/");
   
      fs.readFile(pathOfString, function(err, data) { // this is a callback function!
        var msgBody = Sum Operation Done! ;
        
        // here you could also check if there is a error       
        if (err) { reject(err); }

        // return msgBody;
        // You can not return from the callback so use resolve
        resolve(msgBody);
      });
    });
  }
};

//Class A:
async function myFun(){
  var respondMsg = await readFile.readMSGFile(detailsoffile);
  console.log(respondMsg);
}

Callback way:

//readmsgfile.js
module.exports = {
  readMSGFile: function (filepath, successCallback, failureCallback) {
    let path = filepath;
    var regex = /\\/g;
    const pathOfString = path.replace(regex, "/");
   
    fs.readFile(pathOfString, function (err, data) { // this is a callback function!
      var msgBody = Sum Operation Done! ;

        // here you could also check if there is a error       
        if (err) { failureCallback(err); }

        // return msgBody;
        // You can not return from the callback so use your successCallback
        successCallback(msgBody);
    });
  }
};

//Class A:
function myFun(){
  var respondMsg = readFile.readMSGFile(detailsoffile, msgBody => {
    console.log(respondMsg);
  });
}